Feedback-based selection of regions for abortable garbage collection

ABSTRACT

The disclosed embodiments provide a method, apparatus, and system for selecting, based on feedback from previous garbage collections, a portion of a referenced memory area for garbage collection within a time window. During the execution of a software program, the system selects a given portion of a referenced memory area on which garbage collection can be completed within the given time window and attempts to complete garbage collection on at least the given portion of the referenced memory area before the end of the given time window. Next, the system selects, based on the results of the garbage collection performed during the given time window, a subsequent portion of the referenced memory area on which garbage collection can be completed within the subsequent time window and attempts to complete garbage collection on at least the subsequent portion of the referenced memory area before the end of the subsequent time window.

BACKGROUND

Field

The disclosed embodiments relate to techniques for performing garbagecollection operations to facilitate the execution of a software program.More specifically, the disclosed embodiments relate to techniques forselecting one or more regions of memory for incremental garbagecollection.

Related Art

During the execution of a software program, one or more threads of thesoftware program may dynamically allocate memory on the heap to storeobjects used by the software program. The execution environment of thesoftware program often provides automatic memory management in the formof a garbage collection system. More specifically, a garbage collectormay automatically reclaim memory from dead objects on the heap on behalfof the software program, thereby freeing the software program fromhaving to clean up the heap itself.

However, to avoid jeopardizing the integrity of the software program'sstate, the garbage collector may pause all threads of the softwareprogram while reclaiming memory from the heap. If these pauses in thesoftware program's execution run too long, they may degrade a user'sexperience of the software program. On the other hand, if the pauses aretoo short, garbage collection throughput may be disproportionatelyreduced. Hence, what is needed is a garbage collector implementationthat retains garbage collection throughput during stop-the-world eventsthat are short enough to avoid affecting the software program'susability.

SUMMARY

The disclosed embodiments provide a method, apparatus, and system forselecting, based on feedback from previous garbage collections, aportion of a referenced memory area for garbage collection within a timewindow.

During the execution of a software program, based at least in part ondetermining that the software program is paused for a given time windowwhile garbage collection is being performed on a referenced memory areaof the software program, the system: (1) selects a given portion of thereferenced memory area on which garbage collection can be completedwithin the given time window; and (2) attempts to complete garbagecollection on at least the given portion of the referenced memory areabefore the end of the given time window. Next, based at least in part ondetermining that the software program is paused for a subsequent timewindow while garbage collection is being performed on the referencedmemory area, the system: (1) selects, based at least in part on theresults of the garbage collection performed during the given timewindow, a subsequent portion of the referenced memory area on whichgarbage collection can be completed within the subsequent time window;and (2) attempts to complete garbage collection on at least thesubsequent portion of the referenced memory area before the end of thesubsequent time window.

In some embodiments, the referenced memory area includes a heap.

In some embodiments, the given portion of the referenced memory areaincludes a given set of regions within the heap on which garbagecollection can be completed within the given time window.

In some embodiments, the subsequent portion of the referenced areaincludes a subsequent set of regions within the heap on which garbagecollection can be completed within the subsequent time window.

In some embodiments, attempting to complete garbage collection on atleast an estimated portion of the referenced memory area before the endof a time window includes the following steps. Initially, the systemperforms garbage collection on the portion of the referenced memory.Next, while the time window has not ended, the system determines aremaining amount of time left in the time window and selects, based atleast in part on the results of the garbage collection performed on theestimated portion of the referenced memory, another portion of thereferenced memory on which garbage collection can be completed withinthe remaining amount of time.

In some embodiments, attempting to complete garbage collection on atleast an estimated portion of the referenced memory area before the endof a time window includes aborting the garbage collection on theestimated portion of the referenced memory area if garbage collection onthe portion of the referenced memory area is not completed by the end ofthe time window.

In some embodiments, selecting a portion of the referenced memory areaon which garbage collection can be completed within a time window isbased at least in part on at least one of:

-   -   (i) the results of one or more previous garbage collections; and    -   (ii) the results of a previous part of a current garbage        collection.

In some embodiments, the results of a previous garbage collectionperformed during a time window include information on at least one of:

-   -   (i) whether the previous garbage collection was aborted;    -   (ii) an amount of time taken to abort the previous garbage        collection;    -   (iii) a remaining amount of time left in the time window after        performing the previous garbage collection;    -   (iv) a number of objects collected during the previous garbage        collection; and    -   (v) an average amount of time taken to copy an object during the        previous garbage collection.

In some embodiments, previous garbage collection abortions cause theestimation to be more conservative and previous successful garbagecollection completions cause the estimation to be less conservative.

In some embodiments, the estimation is further based at least in part ondata provided by a user of the software program.

In some embodiments, each of the time windows is based at least in parton a specified maximum time length that the software program can bepaused during the software program's execution.

As described herein, first, second, third, and other ordinal adjectivesare naming conventions that are not necessarily indicative of orderunless otherwise functionally required.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a diagram of a system in accordance with the disclosedembodiments.

FIG. 2 shows a conceptual division of a heap into multiple parts inaccordance with the disclosed embodiments.

FIG. 3 shows a conceptual division of a heap into multiple parts inaccordance with the disclosed embodiments.

FIGS. 4A-4F show successful garbage collections in accordance with thedisclosed embodiments.

FIGS. 5A-5B show an aborted garbage collection attempt in accordancewith the disclosed embodiments.

FIG. 6 shows a flowchart illustrating an exemplary process of garbagecollection in accordance with the disclosed embodiments.

FIG. 7 shows a flowchart illustrating an exemplary process of selectingregions for garbage collection based on feedback in accordance with thedisclosed embodiments.

FIG. 8 shows a computer system in accordance with the disclosedembodiments.

In the figures, like reference numerals refer to the same figureelements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the embodiments, and is provided in the contextof a particular application and its requirements. Various modificationsto the disclosed embodiments will be readily apparent to those skilledin the art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present disclosure. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing code and/or data now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium.

Furthermore, methods and processes described herein can be included inhardware modules or apparatus. These modules or apparatus may include,but are not limited to, an application-specific integrated circuit(ASIC) chip, a field-programmable gate array (FPGA), a dedicated orshared processor that executes a particular software module or a pieceof code at a particular time, and/or other programmable-logic devicesnow known or later developed. When the hardware modules or apparatus areactivated, they perform the methods and processes included within them.

The disclosed embodiments provide a method, apparatus, and system forfacilitating garbage collection within a referenced memory area that isused by a software program. During the execution of the softwareprogram, the results of one or more garbage collections may be recorded.These results may later be used as feedback when selecting a portion ofthe referenced memory area on which garbage collection is to becompleted within a time window.

More specifically, the disclosed embodiments provide a garbage collectorimplementation that is (1) incremental, (2) abortable, and (3) capableof using feedback from past garbage collections, user provided input,and other data to more accurately estimate a number of regions within aheap that can be garbage collected within a stop-the-world event,wherein the event is constrained by a maximum time length.

In general, a software program may rely on a garbage collector providedby the execution environment to reclaim heap memory from dead objectsthat were previously allocated by the software program. To avoidjeopardizing the integrity of the software program's state (e.g., byaccidentally collecting live objects), the garbage collector may pauseall (mutator) threads of the software program while reclaiming memoryfrom the heap. Known as a stop-the-world event, this pause in executionmay degrade the software program's usability if it runs too long,especially if latency is a concern. The necessity of pausing all mutatorthreads of a software program during a garbage collection is especiallyproblematic when dealing with large heaps (e.g., 50 gigabytes (GB) ormore) because the time to process the entire heap grows along with theheap's size.

The impact of stop-the-world events may be mitigated through the use ofan incremental garbage collector that is capable of performing a garbagecollection on a portion of the heap (i.e., perform an incrementalgarbage collection). Here, the portion of the heap that is selected forgarbage collection (e.g., the collection set) may correspond to one ormore regions of the heap. While the incremental garbage collector maystill impose stop-the-world events, the length of one or more of theseevents may no longer directly correspond with the size of the heap.Instead, the incremental garbage collector may limit the duration of thestop-the-world events to less than a maximum time length (that may beprovided by the user of the software program) and attempt to garbagecollect (i.e., process) different portions of the heap during each ofthose durations.

In general, incremental garbage collectors do not know how long it willtake to process a region of the heap. Thus, if an incremental garbagecollector selects a collection set that is too large to process within agiven stop-the-world event, the garbage collector may have to stopand/or abort the garbage collection of the set when time runs out, whichmay be costly and/or unsafe. To avoid this scenario, some incrementalgarbage collectors may: (1) select smaller collection sets; and/or (2)constantly check how much time is left in a stop-the-world event as itprocesses a collection set. However, adopting these strategies mayresult in reduced garbage collection throughput. The disclosedembodiments provide an incremental abortable garbage collector that iscapable of using feedback from past garbage collections to moreaccurately estimate a number of regions that it can process within astop-the-world event of a given duration, thereby improving the softwareprogram's user experience while preserving garbage collectionthroughput.

During the operation of a software program, one or more (mutator)threads of a garbage collector may be halted while one or more of thesoftware program's threads continue to execute within a hostenvironment. When a stop-the-world event occurs, the garbage collectormay initially determine the length of the stop-the-world event. Once itdetermines how much time it has to garbage collect, the garbagecollector may estimate the highest number of regions of the heap (e.g.,10 regions) on which garbage collection can be completed within thestop-the-world event. In making the estimate, the garbage collector mayuse one or more heuristics and/or algorithms that accept data fromvarious sources including (1) previous garbage collection results, (2)data provided by the user, and/or (3) one or more characteristics of thehost environment. The garbage collector may then select a collection setwith a size that is equal to the estimated number of regions and attemptto complete garbage collection on the collection set before the end ofthe stop-the-world event. If the garbage collector is unable to processthe entire collection set by the end of the stop-the-world event, thegarbage collector may abort the garbage collection and record theaborted garbage collection attempt for future reference. Here, theaborted garbage collection attempt may cause the garbage collector tomake a more conservative estimate during a future stop-the-world event.

On the other hand, if the garbage collector processes the entirecollection prior to the end of the stop-the-world event, the garbagecollector may record the successful garbage collection for futurereference, which may cause the garbage collector to make a lessconservative estimate in a future stop-the-world event. Additionally, ifthere is enough time remaining, the garbage collector may attempt toestimate, select, and/or process one or more additional smallercollection sets before the end of stop-the-world event.

Once the stop-the-world event finishes, the garbage collector's threadshalt while the software program's threads resume execution. Once thenext stop-the-world event is imposed on the software program, thegarbage collector may feed the results of the previous garbagecollection into the one or more heuristics and/or algorithms to moreaccurately estimate the highest number of regions of the heap on whichgarbage collection can be completed within the latest stop-the-worldevent.

It should be noted that in some embodiments, the garbage collector mayestimate at the object level rather than the region level. Morespecifically, during a stop-the-world event, the garbage collector maydetermine the highest number of objects (e.g., 10,000 objects) that canbe collected within the stop-the-world event instead of the highestnumber of regions. This particular variation may be advantageous insituations where more granularity with regards to incremental garbagecollection is desired.

FIG. 1 shows a diagram of a system in accordance in with the disclosedembodiments. As shown in FIG. 1, software program 104 may execute onvirtual machine 102. Virtual machine 102 may execute on top of operatingsystem 106 and/or computing hardware 108, all of which are containedwithin host environment 100 of the host system. More specifically,during software program 104's execution, program classes 130 of softwareprogram 104 may be loaded by class loader subsystem 110 into runtimedata areas 112. Additionally, one or more components of execution engine114 may operate on data present in runtime data areas 112. Thesecomponents may include Just-in-Time (JIT) compiler 126 and garbagecollector 128.

Host environment 100 may correspond to a stack of one or more softwarecomponents that execute on computing hardware 108, which may correspondto a smartphone, a cell phone, a personal digital assistant (PDA), alaptop computer, a desktop computer, a tablet computer, a server, acluster of servers, a hypervisor, a container, or an embedded system.

Software program 104 may be an application that executes on an operatingsystem, an embedded application that executes on firmware, an enterpriseapplication, a database, a shared library, a device driver, and/or anyother type of software program. Software program 104 may be written in ahigh-level programming language that can be compiled into anintermediate code via a compiler. Additionally, software program 104, inits intermediate code form, may be configured to execute on virtualmachine 102. For example, software program 104 may be written in theJava programming language; virtual machine 102 may follow the JavaVirtual Machine (JVM) specification; and software program 104 may becompiled into Java byte code, which may be executed on any softwareenvironment that includes a JVM. Additionally, software program 104 maybe composed of one or more classes that specifically implementfunctionality of the software program, which are represented by programclasses 130. For example, program classes 130 may each correspond to aclass written in a high-level programming language (e.g., Java) that hasbeen compiled to an intermediate code (e.g., Java byte code).

Virtual machine 102 may correspond to any virtual machine that providesone or more data areas for storing software program 104 in itsintermediate code form at runtime and an interpreter and/or ajust-in-time (JIT) compiler for executing the loaded intermediate code.A virtual machine may be referred to as a software implementation of amachine that executes programs like a physical machine. Examples of JVMimplementations include Oracle Hotspot JVM and International BusinessMachines (IBM) JVM. In particular, virtual machine 102 may include classloader subsystem 110, runtime data areas 112, and execution engine 114.

Class loader subsystem 110 may correspond to a component of virtualmachine 102 that handles the loading of program classes 130, which havebeen compiled into intermediate code, into one or more data areas of thevirtual machine at runtime.

Runtime data areas 112 may correspond to one or more referenced memoryareas organized by virtual machine 102 to hold the intermediate code ofsoftware program 104 and the data it operates upon during execution. Inparticular, the runtime data areas provided by virtual machine 102 toexecute software program 104 may include heap 120 and stacks 122. Incases where software program 104 is multi-threaded, some components ofruntime data areas 112 are shared among all threads while others areunique to individual threads. One stack exists for each thread. Thestack is created when the thread starts. Thus, stacks 122 may refer toall stacks created and assigned to the threads of software program 104.A thread's stack may store the state of one or more current methodinvocations for the thread in one or more stack frames. A stack frame iscreated and pushed onto a thread's stack when a method is executed invirtual machine 102 by the thread. When the method finishes, the stackframe is popped off the stack. In particular, a stack frame contains oneor more components of the method's state, which may include one or morelocal variables, one or more parameters with which the method wasinvoked, one or more return values, and intermediate calculations. Somelocal variables and/or parameters may hold primitive data values, whichare stored in the stack frame. Other local variables and/or parametersmay hold references to objects, which are created and stored on heap120. Thus, object references stored in all stack frames of all stacksfor a software program may be referred to as the “root set” of thesoftware program. In some embodiments, the root set may additionallyinclude any object references, such as strings, in a constant pool ofloaded classes. The constant pool of a loaded class may refer to stringsstored on the heap, such as the class name, superclass name,superinterface names, field names, field signatures, method names, andmethod signatures.

Heap 120 may correspond to a heap, which is a referenced memory areathat stores objects created during the execution of a software program.Unlike stacks, however, a heap may be shared by all threads. Whenever athread creates an object, the object is stored in a portion of memoryallocated on the heap while a reference to the object is created andstored in the current stack frame of the thread. This reference to theobject may be passed from method to method within the thread and evenshared among multiple threads within their stack frames. Threads maythen use these references to access the object and modify the object onthe heap. Additionally, other objects that are allocated on the heap maypossess references to the object. So long as the object is reachablefrom the software program's root set (e.g., directly by a thread througha reference stored on its stack, or indirectly through one or more otherobjects on the heap), the object is known as a live object (i.e., anobject with liveness). Objects that are unreachable from the softwareprogram are considered garbage and are the target of garbage collection.Heaps are discussed in further detail below with respect to FIGS. 2-3.

Execution engine 114 may correspond to a component of virtual machine102 that executes the intermediate code, which is loaded into runtimedata areas 112 by class loader subsystem 110. In particular, theexecution engine may employ JIT compiler 126 to periodically compileportions of the intermediate code into native code, which is thendirectly executed by execution engine 114. Additionally, executionengine 114 may provide garbage collector 128.

Garbage collector 128 may correspond to an incremental garbage collectorused by execution engine 114 to manage memory in heap 120 during theexecution of software program 104. Functions performed by garbagecollectors may include: (1) automatically clearing garbage (i.e.,unreachable objects) from the heap; and (2) moving objects betweensections of the heap to reduce heap fragmentation. A software programmay constantly create and store objects in heap memory. Garbagecollectors may prevent the heap from running out of memory byautomatically reclaiming objects that are unreachable from the softwareprogram's root set. Additionally, free space within the heap may becomefragmented due to objects being allocated and reclaimed at variouslocations throughout the heap. To help ensure that the heap can providelarge contiguous blocks of memory with which to store larger objects ofthe software program, the garbage collector may periodically compact alllive objects into alternate sections of the heap, thereby combiningsmall pockets of free heap space into larger ones. To accomplish thesetasks more efficiently, the garbage collector may conceptually dividethe heap into (1) multiple generations, and into (2) multiple regions,as discussed in further detail below with respect to FIGS. 2-3.

FIGS. 2-3 both show conceptual divisions of a heap into multiple partsin accordance with the disclosed embodiments. More specifically, FIG. 2displays how a garbage collector may conceptually divide a heap intomultiple generations so that dead objects can be collected moreefficiently. FIG. 3 displays how the garbage collector may also divide aheap into multiple regions, wherein each region is assigned to one ofthe generations.

Garbage collector 128 may execute via one or more threads that operateon data stored in runtime data areas 112. To perform certain tasks,garbage collector 128 may invoke one or more stop-the-world eventsduring the duration of software program 104's execution. During astop-the-world event, all (mutator) threads of software program 104 arepaused, allowing threads of garbage collector 128 to complete tasks thatcannot be interrupted without risking the integrity of the softwareprogram's runtime state. If the heap is large, stop-the-world events canhalt a software program for seconds, which can affect the usability ofthe software program. Thus, garbage collector 128 may divide heap 120into multiple generations, which include young generation 202 and oldgeneration 204. Young generation 202 may correspond to a part of heap120 that stores young objects, while old generation 204 may correspondto another part of the heap that stores old (tenured) objects. Inparticular, the “age” of an object refers to the number of garbagecollections the object has survived.

Because objects that have survived a threshold number of garbagecollections (i.e., older objects) are less likely to yield free spaceduring future garbage collections, garbage collector 128 may: (1) keepold objects in old generation 204 and young objects in young generation202; and (2) perform garbage collections more frequently on younggeneration 202 (i.e., perform an incremental garbage collection withyoung generation 202 as the collection set) and performing garbagecollections less frequently on old generation 204. In some embodiments,a general garbage collector may divide heap 120 into any number ofgenerations.

Young generation 202 may be further divided into the following parts:Eden space 212, first survivor space 214, and second survivor space 216.Newly created objects may initially be stored in Eden space 212. Duringan incremental garbage collection, garbage collector 128 may move livingobjects from Eden space 212 and first survivor space 214 into secondsurvivor space 216 and clear spaces 212-214 of garbage. During asubsequent incremental garbage collection, garbage collector 128 maymove living objects from Eden space 212 and second survivor space 216back into first survivor space 214, before clearing spaces 212 and 216of garbage.

To more efficiently conduct garbage collection over heap 120, garbagecollector 128 may divide heap 120 into a plurality of equally sizedregions (regions 208) and assign each region to either young generation202 or old generation 204. For example, a 50 GB heap may be subdividedinto a number of 32 MB (megabyte) regions. As shown in FIG. 3, part ofheap 120 may be allocated to form a plurality of regions, while the restof the heap may exist as free space from which new regions may beallocated on demand. In particular, Eden space 212 may be composed ofthe regions that contain an ‘E’, first survivor space 214 may becomposed of the regions that contain “S1”, second survivor space 216 maybe composed of the regions that contain “S2”, and old generation 204 maybe composed of the regions that contain an ‘O’.

To preserve the runtime state of software program 104 during eachincremental garbage collection, garbage collector 128 may move and/orcopy out all living objects within a region prior to garbage collectingthat region. Living objects may include objects that garbage collector128 can reach by following one or more references from software program104's root set. After moving and/or copying out these objects, garbagecollector 128 then garbage collects all other objects within thecollection set.

It should also be noted that garbage collector 128 may allocate memoryfrom heap 120 to store auxiliary data structures 206, which may includeone or more remembered sets. Although FIG. 2 depicts auxiliary datastructures 206 as being stored in a separate part of the heap, otherways of organizing the auxiliary data structures in heap 120 may be used(e.g., storing remembered sets within each region of the heap).

FIGS. 4A-4F show a number of successful garbage collections inaccordance with the disclosed embodiments. More specifically FIGS. 4A-4Fillustrate a number of interactions between a garbage collector and theheap while the garbage collector attempts to process a number of regionsof the heap during a time window provided by a stop-the-world event.

FIG. 4A shows selection apparatus 404, which may be included withingarbage collector 128, selecting regions within heap 120 at thebeginning of stop-the-world event 402. In particular, the selectionapparatus receives a number of input parameters, including thestop-the-world event's length and previous garbage collection results406.

Stop-the-world event 402 may be imposed on software program 104 inresponse to garbage collector 128 determining that free space withinheap 120 has reached or gone below a particular threshold. Depending onthe state of the heap, the stop-the-world event may be part of a younggeneration garbage collection or an old (mixed) generation garbagecollection.

Previous garbage collection results 406 may include the results of oneor more garbage collections that were previously attempted on heap 120.Some of these past garbage collections may be successful garbagecollections. A successful garbage collection may correspond to an eventwhere the garbage collector succeeds in processing, within the timewindow provided by the stop-the-world event, the regions that wereselected by the selection apparatus. Some of these garbage collectionsmay have been aborted. An aborted garbage collection may correspond toan event where the garbage collector is not able to process, within thetime window provided by the stop-the-world event, all regions that wereselected in that garbage collection and has to abort the garbagecollection. In some embodiments, previous garbage collection results 406may be stored within any type of data structure such as an array and/ora linked list. In some embodiments, the results may be stored within adatabase and/or an external file.

Results for a given garbage collection may include (1) information onwhether the garbage collection was successful or aborted, (2) (ifaborted) the amount of time it took to abort the garbage collection, (3)(if successful) the amount of time it took to perform the garbagecollection, (4) the amount of time remaining in the stop-the-world eventafter the garbage collection was completed, (5) the number of regions inthe collection set, (6) the total number of objects marked during thegarbage collection, (7) the total number of objects collected during thegarbage collection, and/or (8) the average amount of time needed to copyan object to the survivor space of the garbage collection.

As shown in FIG. 4A, the length of the time window provided by thestop-the-world event may be 200 milliseconds (ms). Additionally, becausethe stop-the-world event has just started, there is 200 ms of timeremaining in the stop-the-world event. At this point, selectionapparatus 404 may estimate a number of regions within heap 120 on whichgarbage collection can be completed (i.e., can be evacuated) within theremaining time. In making the estimation, selection apparatus 404 mayuse information stored within the results of previous garbagecollections as feedback.

The selection apparatus may consider the number of previous successfulgarbage collections and/or the number of aborted garbage collectionsrecorded during the execution of software program 104. Here, an abortedgarbage collection may indicate that past estimates were too high and/orinaccurate and that the selection apparatus 404 should be moreconservative in estimating the number of regions that can be processedwithin the remaining time. On the other hand, a successful garbagecollection (especially one with a relatively large amount of timeremaining in the associated stop-the-world event) may indicate that pastestimates were too low and that less conservative estimates should bemade. In some embodiments, the selection apparatus may place more weighton more recent garbage collections. For example, if garbage collector128 successfully completes ten garbage collections early in softwareprogram 104's execution but aborts the last three garbage collections(e.g., because regions may accumulate more objects as the softwareprogram executes), the selection apparatus may make a more conservativeestimate despite the higher number of successful garbage collections.

The selection apparatus may also consider how long it takes to abort agarbage collection. An average amount of time needed to abort a garbagecollection may be calculated using time measurements from previousaborted garbage collections. In some embodiments, when calculating theaverage, the selection apparatus may give more weight to recent abortedgarbage collections. If the average amount of time needed to abort froma garbage collection is high, selection apparatus 404 may make moreconservative estimates. On the other hand, if the average amount of timeneeded to abort from a garbage collection is low, selection apparatus404 may make less conservative estimates.

In some embodiments, the selection apparatus may also consider auser-settable conservativeness factor, wherein a higher conservativefactor would cause the selection apparatus to make more conservativeestimates while a lower conservative rating would cause the selectionapparatus to prioritize garbage collection throughput. Thisconservativeness factor may be supplied by the user via a command-lineargument and/or a configuration file to software program 104.

In some embodiments, the selection apparatus may consider the amount offree space left in heap 120. In particular, the conservativeness ofestimates made by the selection apparatus may be inversely correlatedwith the amount of free space left in the heap.

In some embodiments, to estimate the number of regions that can beprocessed within the stop-the-world event's remaining time, theselection apparatus may first determine the average amount of timeneeded to perform a garbage collection on a single region of heap 120.To do so, the selection apparatus may consider (1) how much garbage isin each region, (2) how many objects needed to be copied out of eachregion, and (3) how long it takes to copy an object to the survivorspace of the garbage collection. For example, the average may bedetermined by multiplying the average number of objects found in eachregion by the average amount of time needed to move an object to thesurvivor space. Once the average amount of time needed to perform agarbage collection on a single region is determined, the selectionapparatus may determine the highest number whose product with theaverage amount of time is less than the remaining time of thestop-the-world event. In some embodiments, the selection apparatus mayapply a probability factor to one or more previous estimates to make thecurrent estimate. In some embodiments, the selection apparatus may applysmoothing filters to one or more previous estimates to make the currentestimate.

Once the estimated number of regions is obtained, the selectionapparatus selects the number of regions from the heap as the collectionset. For example, the selection apparatus may estimate that five regionscan be processed within 200 ms and then select five regions from heap120 as the collection set.

FIG. 4B shows the storing of results from the first garbage collection(i.e., the first evacuation), which was successfully completed duringstop-the-world event 402. As shown in FIG. 4B, the five-regioncollection set has been processed within 150 ms, leaving 50 ms of timeremaining in the stop-the-world event. At this point, results of thefive-region garbage collection may be stored within previous garbagecollection results 406 to be used as feedback for future garbagecollections. In particular, results of the five-region garbagecollection may include (1) the fact that the garbage collection wascompleted successfully, (2) the time remaining after the garbagecollection was completed, (3) the number of regions collected, (4) thenumber of objects marked and/or collected per region processed, and (5)the average amount of time used to copy an object to the survivor spaceof the garbage collection. Additionally, the results of the successfulgarbage collection may cause selection apparatus 404 to make lessconservative estimates during future stop-the-world events.

FIG. 4C shows an attempt to perform a second (smaller) garbagecollection in response to a first check performed during stop-the-worldevent 402. During the check, if garbage collector 128 determines thatthere is still time remaining within a stop-the-world event after thefirst garbage collection is performed, it may attempt to perform anadditional garbage collection (i.e., a second evacuation) within theremaining time to increase garbage collection throughput.

In particular, selection apparatus 404 may estimate that two moreregions can be processed within the remaining 50 ms of thestop-the-world event and select two regions (as shown by the two moreheavily bordered squares in heap 120) as a second collection set to beprocessed within the stop-the-world event. The second estimate may beobtained using the same algorithms and/or heuristics that were used tocalculate the first estimate of FIG. 4A. Additionally, while making thesecond estimate, selection apparatus 404 may use: (1) the results of thefive-region garbage collection; and (2) other results stored prior tothe five-region garbage collection as feedback.

FIG. 4D shows the storing of results from the second garbage collection(i.e., the second evacuation) that is successfully completed duringstop-the-world event 402. As shown in FIG. 4D, the two-region collectionset has been processed within 47 ms, leaving 3 ms of time remainingwithin the stop-the-world event. Here, the results of the two-regiongarbage collection may be stored within previous garbage collectionresults 406 to be used as feedback for future garbage collections. Inparticular, results of the two-region garbage collection may includesimilar information included in the results of the five-region garbagecollection. Additionally, the results from the successful garbagecollection may cause selection apparatus 404 to make less conservativeestimates during future stop-the-world events.

At this point, because there is only 3 ms of remaining time left instop-the-world even 402, selection apparatus 404 may determine thatthere is not enough time to process another region of the heap. As aresult, garbage collector 128 may forgo attempting to garbage collectany more regions until the next stop-the-world event arrives. Once theremaining time runs out, software program 104 resumes execution whilethreads of garbage collector 128 are halted until the nextstop-the-world event. In some embodiments, rather than wait for the 3 msto run out, garbage collector 128 may prematurely end stop-the-worldevent 402, thereby allowing software program 104 to resume earlier.

FIG. 4E shows an attempt to perform the first garbage collection duringa subsequent stop-the-world event (stop-the-world event 412). Here,selection apparatus 404 may estimate the highest number of regions onwhich garbage collection can be completed within stop-the-world event412, which is also 200 ms long. In doing so, however, the selectionapparatus may use (1) results of the two-region garbage collectionstored in FIG. 4D, (2) results of the five-region garbage collectionstored in FIG. 4B, and (3) other results stored prior to the five-regiongarbage collection as feedback. In particular, selection apparatus 404may consider that a total of seven regions were successfully processedwithin 200 ms during stop-the-world event 412. While this is noguarantee that garbage collector 128 can process a single seven-regioncollection set within the 200 ms time window (because certain regionsmay contain more and/or larger objects to mark and/or copy), selectionapparatus 404 may estimate that seven regions can be collected withinstop-the-world event 412. The selection apparatus may then select aseven-region collection set for the next garbage collection attempt.

FIG. 4F shows the storing of results from the first garbage collectionthat is successfully completed during stop-the-world event 412. As shownin FIG. 4F, the seven-region collection set has been collected within193 ms. Additionally, because there is only 7 ms remaining withinstop-the-world event 412, selection apparatus 404 may determine thatthere is not enough time to attempt a second garbage collection duringthe stop-the-world event. As a result, garbage collector 128 may notattempt to garbage collect any more regions until the nextstop-the-world event arrives. Once the remaining time runs out, softwareprogram 104 resumes execution while the threads of garbage collector 128are halted until the next stop-the-world event.

It should be noted that the first garbage collection (where five regionsare evacuated) and the additional garbage collection (where twoadditional regions are evacuated) mentioned in FIGS. 4A-4D may bereferred to as a first evacuation phase and a second evacuation phaserespectively, wherein the two evacuation phases are part of a singlegarbage collection illustrated in FIGS. 4A-4D. In particular, thegarbage collection illustrated in FIGS. 4A-4D contains two evacuationphases (the first evacuation phase shown in FIGS. 4A-4B and the secondevacuation phase shown in FIGS. 4C-4D) because there was enough timeafter the first evacuation phase to perform a second smaller evacuation(i.e., the additional garbage collection). In contrast, the garbagecollection depicted in FIGS. 4E-4F may contain only a single evacuationphase. Here, the phrase “garbage collection” has been used to refer toboth evacuation phases, which are performed within a time window (whichmay be a portion of a stop-the-world event), and garbage collections,which may contain multiple evacuations phases and are performed within acomplete stop-the-world event.

FIGS. 5A-5B show an aborted garbage collection attempt in accordancewith the disclosed embodiments. More specifically FIGS. 5A-5B illustratea number of interactions between a garbage collector and the heap whilethe garbage collector attempts to collect an overly large collection setfrom the heap during a stop-the-world event.

FIG. 5A shows an attempt to perform a garbage collection duringstop-the-world event 502. Here, selection apparatus 404 may estimate thehighest number of regions on which garbage collection can be completedwithin stop-the-world event 502, which is also 200 ms long. Whenselection apparatus 404 estimates how many regions should be collectedwithin 200 ms, it may obtain feedback from previous garbage collectionsstored in previous garbage collection results 504. Here, selectionapparatus 404 estimates that nine regions can be processed within 200ms. As a result, the selection apparatus selects a nine-regioncollection set for processing.

FIG. 5B shows the storing of results from a garbage collection that isaborted during stop-the-world event 502. As shown in heap 120, onlyseven regions of the nine-region collection set have been processed.Because there is no time left in stop-the-world event 502 for processingthe last two selected regions, garbage collector 128 may abort thisgarbage collection. In some embodiments, garbage collector 128 may needto roll back any operations performed (e.g., restoring collectedobjects, deleting copied objects, etc.) on at least one of the sevenprocessed regions to preserve the integrity of software program 104'sstate. In other embodiments, garbage collector 128 may not need to rollback any operations. In this case, the seven regions may be consideredprocessed. It should also be noted that in some embodiments, garbagecollector 128 may abort a garbage collection even if there is time leftbut not enough time to perform a meaningful amount of work (e.g., 3-4ms).

Additionally, results of the aborted garbage collection attempt may bestored within previous garbage collection results 504 to be used asfeedback for future garbage collections. In particular, results of theaborted garbage collection attempt may include (1) the fact that thegarbage collection was aborted, (2) the amount of time used to abort thegarbage collection attempt, (3) the number of regions collected, (4) thenumber of regions selected, (5) the number of objects marked and/orcollected per region processed, and (6) the average amount of time usedto copy an object to the survivor space of the garbage collection.Additionally, the results from the aborted garbage collection attemptmay cause selection apparatus 404 to make more conservative estimatesduring future stop-the-world events.

FIG. 6 shows a flowchart illustrating an exemplary process of garbagecollection in accordance with the disclosed embodiments. In one or moreembodiments, one or more of the steps may be omitted, repeated, and/orperformed in a different order. Accordingly, the specific arrangement ofsteps shown in FIG. 6 should not be construed as limiting the scope ofthe embodiments.

During the initialization of a software program, the garbage collectorprovided by the host environment in which the software program executesreceives a maximum pause time (operation 602). In some embodiments, themaximum pause time may be received from a user of the software programvia a command line argument and/or a configuration file that isassociated with the software program. In general, the host environmentmay try to ensure that garbage collection efforts do not interrupt theexecution of the software program for a time period longer than themaximum pause time. In other words, stop-the-world events that areimposed on the software program and time windows during which garbagecollections may be attempted and/or completed may be less than or equalto the maximum pause time. In some embodiments, however, forparticularly intensive garbage collections (e.g., old/mixed garbagecollections), stop-the-world events and time windows that are longerthan the maximum pause time may be imposed. During the execution of thesoftware program, one or more threads of the garbage collector may bepaused (operation 604). When the amount of free space within a heap thatis used by the software program falls below a particular threshold, agarbage collection event may be triggered. Next, the length of astop-the-world event may be dynamically determined based on the maximumpause time (operation 606). In most cases, the length of stop-the-worldevent will be either equal to or shorter than the maximum pause time. Insome embodiments, the length of all stop-the-world events may be astatic value that is set, based on the maximum pause time, at thebeginning of the software program's execution. The stop-the-world eventis then imposed on the software program. During the stop-the-worldevent, the garbage collector performs garbage collection on the heapwhile the software program's threads are paused (operation 608).

FIG. 7 shows a flowchart illustrating an exemplary process of selectingregions for garbage collection based on feedback in accordance with thedisclosed embodiments. In one or more embodiments, one or more of thesteps may be omitted, repeated, and/or performed in a different order.Accordingly, the specific arrangement of steps shown in FIG. 7 shouldnot be construed as limiting the scope of the embodiments.

During the stop-the-world event, the garbage collector may firstdetermine the length of the stop-the-world event (operation 702). Next,based on the length of the stop-the-world event and feedback fromprevious garbage collections, the garbage collector may estimate anumber of regions of the heap on which garbage collection can becompleted and select that number of regions of the heap for garbagecollection (operation 704). The garbage collector may then attempt tocomplete garbage collection on the selected regions of the heap withinthe stop-the-world event (operation 706). If the garbage collector runsout of time before it can finish processing the selected regions(decision 708), the garbage collector may abort the garbage collection(operation 714) and record the aborted garbage collection attempt(operation 716), which may later be used as feedback for future garbagecollections. On the other hand, if the garbage collector is able tofinish processing the selected regions within the stop-the-world event(decision 708), the garbage collector may record the successful garbagecollection (operation 710), which may later be used as feedback forfuture garbage collections. Next, the garbage collector may determinewhether there is enough time left within the stop-the-world event toattempt another garbage collection (decision 712). If so, the garbagecollector may attempt another garbage collection within the remainingtime (operations 702-706). Once the stop-the-world event ends, thesoftware program resumes execution while the garbage collector's threadsare halted until the next stop-the-world event.

FIG. 8 shows a computer system 800 in accordance with an embodiment.Computer system 800 may correspond to an apparatus that includes aprocessor 802, memory 804, storage 806, and/or other components found inelectronic computing devices such as personal computers, laptopcomputers, workstations, servers, mobile phones, tablet computers,and/or portable media players. Processor 802 may support parallelprocessing and/or multi-threaded operation with other processors incomputer system 800. Computer system 800 may also include input/output(I/O) devices such as a keyboard 808, a mouse 810, and a display 812.

Computer system 800 may include functionality to execute variouscomponents of the present embodiments. In particular, computer system800 may include an operating system (not shown) that coordinates the useof hardware and software resources on computer system 800, as well asone or more applications that perform specialized tasks for the user. Toperform tasks for the user, applications may obtain the use of hardwareresources on computer system 800 from the operating system, as well asinteract with the user through a hardware and/or software linkerprovided by the operating system.

In one or more embodiments, computer system 800 provides an incrementaland abortable garbage collection mechanism that uses feedback fromprevious garbage collections to estimate a portion of a referencedmemory area on which garbage collection can be completed within astop-the-world event, wherein the garbage collection mechanism attemptsto process the estimated portion within the stop-the-world event.

In addition, one or more components of computer system 800 may beremotely located and connected to the other components over a network.Portions of the present embodiments (e.g., storage apparatus, extractionapparatus, etc.) may also be located on different nodes of a distributedsystem that implements the embodiments. For example, the presentembodiments may be implemented using a cloud computing system thatimproves the knowledge and management of memory consumption in a set ofremote software programs.

The foregoing descriptions of various embodiments have been presentedonly for purposes of illustration and description. They are not intendedto be exhaustive or to limit the present invention to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present invention.

What is claimed is:
 1. A computer-implemented method, comprising: duringan execution of a software program: based at least in part ondetermining that the software program is paused for a given time windowwhile garbage collection is being performed on a referenced memory areaof the software program: selecting a given portion of the referencedmemory area on which garbage collection can be completed within thegiven time window; and attempting to complete garbage collection on atleast the given portion of the referenced memory area before the end ofthe given time window; and based at least in part on determining thatthe software program is paused for a subsequent time window whilegarbage collection is being performed on the referenced memory area:selecting, based at least in part on the results of the garbagecollection performed during the given time window, a subsequent portionof the referenced memory area on which garbage collection can becompleted within the subsequent time window; and attempting to completegarbage collection on at least the subsequent portion of the referencedmemory area before the end of the subsequent time window.
 2. Thecomputer-implemented method of claim 1: wherein the referenced memoryarea comprises a heap; wherein the given portion of the referencedmemory area comprises a given set of regions within the heap on whichgarbage collection can be completed within the given time window; andwherein the subsequent portion of the referenced area comprises asubsequent set of regions within the heap on which garbage collectioncan be completed within the subsequent time window.
 3. Thecomputer-implemented method of claim 1, wherein attempting to completegarbage collection on at least an estimated portion of the referencedmemory area before the end of a time window comprises: performinggarbage collection on the portion of the referenced memory; and whilethe time window has not ended: determining a remaining amount of timeleft in the time window; and selecting, based at least in part on theresults of the garbage collection performed on the estimated portion ofthe referenced memory, another portion of the referenced memory on whichgarbage collection can be completed within the remaining amount of time.4. The computer-implemented method of claim 1, wherein attempting tocomplete garbage collection on at least an estimated portion of thereferenced memory area before the end of a time window comprises: ifgarbage collection on the portion of the referenced memory area is notcompleted by the end of the time window, aborting the garbage collectionon the estimated portion of the referenced memory area.
 5. Thecomputer-implemented method of claim 4, wherein selecting a portion ofthe referenced memory area on which garbage collection can be completedwithin a time window is based at least in part on at least one of: theresults of one or more previous garbage collections; and the results ofa previous part of a current garbage collection.
 6. Thecomputer-implemented method of claim 5, wherein the results of aprevious garbage collection performed during a time window compriseinformation on at least one of: whether the previous garbage collectionwas aborted; an amount of time taken to abort the previous garbagecollection; a remaining amount of time left in the time window afterperforming the previous garbage collection; a number of objectscollected during the previous garbage collection; and an average amountof time taken to copy an object during the previous garbage collection.7. The computer-implemented method of claim 5: wherein previous garbagecollection abortions cause the estimation to be more conservative; andwherein previous successful garbage collection completions cause theestimation to be less conservative.
 8. The computer-implemented methodof claim 5, wherein the estimation is further based at least in part ondata provided by a user of the software program.
 9. Thecomputer-implemented method of claim 1, wherein each of the time windowsis based at least in part on a specified maximum time length that thesoftware program can be paused during the software program's execution.10. An apparatus, comprising: one or more processors; and memory storinginstructions that, when executed by the one or more processors, causethe apparatus to: during an execution of a software program: based atleast in part on determining that the software program is paused for agiven time window while garbage collection is being performed on areferenced memory area of the software program: select a given portionof the referenced memory area on which garbage collection can becompleted within the given time window; and attempt to complete garbagecollection on at least the given portion of the referenced memory areabefore the end of the given time window; and based at least in part ondetermining that the software program is paused for a subsequent timewindow while garbage collection is being performed on the referencedmemory area: select, based at least in part on the results of thegarbage collection performed during the given time window, a subsequentportion of the referenced memory area on which garbage collection can becompleted within the subsequent time window; and attempt to completegarbage collection on at least the subsequent portion of the referencedmemory area before the end of the subsequent time window.
 11. Theapparatus of claim 10: wherein the referenced memory area comprises aheap; wherein the given portion of the referenced memory area comprisesa given set of regions within the heap on which garbage collection canbe completed within the given time window; and wherein the subsequentportion of the referenced area comprises a subsequent set of regionswithin the heap on which garbage collection can be completed within thesubsequent time window.
 12. The apparatus of claim 10, whereinattempting to complete garbage collection on at least an estimatedportion of the referenced memory area before the end of a time windowcomprises: performing garbage collection on the portion of thereferenced memory; and while the time window has not ended: determininga remaining amount of time left in the time window; and selecting, basedat least in part on the results of the garbage collection performed onthe estimated portion of the referenced memory, another portion of thereferenced memory on which garbage collection can be completed withinthe remaining amount of time.
 13. The apparatus of claim 10, whereinattempting to complete garbage collection on at least an estimatedportion of the referenced memory area before the end of a time windowcomprises: if garbage collection on the portion of the referenced memoryarea is not completed by the end of the time window, aborting thegarbage collection on the estimated portion of the referenced memoryarea.
 14. The apparatus of claim 13, wherein selecting a portion of thereferenced memory area on which garbage collection can be completedwithin a time window is based at least in part on at least one of: theresults of one or more previous garbage collections; and the results ofa previous part of a current garbage collection.
 15. The apparatus ofclaim 14, wherein the results of a previous garbage collection performedduring a time window comprise information on at least one of: whetherthe previous garbage collection was aborted; an amount of time taken toabort the previous garbage collection; a remaining amount of time leftin the time window after performing the previous garbage collection; anumber of objects collected during the previous garbage collection; andan average amount of time taken to copy an object during the previousgarbage collection.
 16. The apparatus of claim 14: wherein previousgarbage collection abortions cause the estimation to be moreconservative; and wherein previous successful garbage collectioncompletions cause the estimation to be less conservative.
 17. Theapparatus of claim 14, wherein the estimation is further based at leastin part on data provided by a user of the software program.
 18. Theapparatus of claim 10, wherein each of the time windows is based atleast in part on a specified maximum time length that the softwareprogram can be paused during the software program's execution.
 19. Oneor more non-transitory computer-readable storage media storinginstructions that when executed by a computer cause the computer toperform a method, the method comprising: during an execution of asoftware program: based at least in part on determining that thesoftware program is paused for a given time window while garbagecollection is being performed on a referenced memory area of thesoftware program: selecting a given portion of the referenced memoryarea on which garbage collection can be completed within the given timewindow; and attempting to complete garbage collection on at least thegiven portion of the referenced memory area before the end of the giventime window; and based at least in part on determining that the softwareprogram is paused for a subsequent time window while garbage collectionis being performed on the referenced memory area: selecting, based atleast in part on the results of the garbage collection performed duringthe given time window, a subsequent portion of the referenced memoryarea on which garbage collection can be completed within the subsequenttime window; and attempting to complete garbage collection on at leastthe subsequent portion of the referenced memory area before the end ofthe subsequent time window.
 20. The non-transitory computer-readablestorage media of claim 19: wherein the referenced memory area comprisesa heap; wherein the given portion of the referenced memory areacomprises a given set of regions within the heap on which garbagecollection can be completed within the given time window; and whereinthe subsequent portion of the referenced area comprises a subsequent setof regions within the heap on which garbage collection can be completedwithin the subsequent time window.